The objective of the proposed studies is to use somatic cell genetics to understand the mechanism of the regulation of cell growth and the control of a gene expression in mammalian cells. We have chosen cultred rat pituitary tumor cells as a model system for this approach because the growth of these cells is regulated by a number of signals. In addition, the expression of a specific gene product, growth hormone, is regulated by many of the same signals. The latter property of these cells is particularly useful because, in parallel with effects on growth, highly specific biochemical probes measuring specific messenger RNA levels and specific gene copy number are available to facilitate analyses of the nature of mutant defects. To isolate hormonally unresponsive cell lines, two properties of these cells will be exploited. The first is the stimulation of cell growth by L-triiodothyronine (T3). If enhancement of growth requires the induction of a specific gene product, unresponsive cells could have mutations throughout the T3-mediated pathway. The second property upon which mutant isolation relies is the synthesis and secretion of growth hormone. Procedures will be developed to screen for the production of this polypeptide from 10 to the 5th power to 10 to the 7th power clones. Because the screening method is quantitative for growth hormone, mutations may be obtained at any of the steps leading to the induction, processing and secretion of the polypeptide. For a complete genetic analysis, the important feature of both selection methods is an approach described that will allow for selection of revertants, which will be essential in an eventual complementation analysis by somatic cell hybridization to determine the number of gene products, and their regulatory function, involved in T3-mediated regulation of growth and gene expression. The biochemical defect suffered in each mutant will be thoroughly characterized with respect to: any protein detected on two-dimensional gel analysis; T3 receptor function; and the synthesis and secretion of growth hormone or its mRNA. The availability of probes for messenger RNA levels and gene copy number will provide additional help for analyzing the nature of mutant deficiencies.